In analyzing samples in spectroscopy, light is passed through an analysis point in which it interacts with the sample placed at that point causing either the absorption of said light or the emission of a secondary light (such as Raman, fluorescence, etc.) by the sample. Both, the degree to which the light is absorbed, and the intensity and spectral characteristics of the emitted secondary light are influenced by the nature of the sample present in the analysis point. In this way a sample can be identified, the composition of a mixture quantified, etc. In some cases the absorption of light or the secondary emitted light are too weak to be reliably measured. One way this was traditionally addressed was by passing light multiple times through the sample using so called multipass cells.
It is common in so called attenuated total reflection (ATR) spectroscopy [N. J. Harrick: Internal Reflection Spectroscopy, Harrick Scientific Corporation, Ossining N.Y., 1987.] to employ a multipass cell comprising an optical element that has two parallel surfaces through which light propagates by reflecting in a zigzag fashion between said surfaces. If an absorbing sample is pressed against one or both of the flat surfaces, the attenuation of light that occurs at a single reflection is magnified by the multiple reflections. Although the effect is thus magnified, in each of these multiple reflections light interacts with a different portion of the sample requiring a large quantity of the sample for analysis. This can be a problem in those cases where only a small amount of sample is available.
Another example of a multipass cell is the so called White cell [John U. White, “Long Optical Paths of Large Aperture”, J. Opt. Soc. Am, No. 32 (1942), pp 285-288] routinely used for the analysis of gases by transmission spectroscopy. Light enters the cell and is reflected between a special arrangement of three spherical mirrors a large number of times until it exits the cell. The absorption of light by the gas in the cell is enhanced by the extended path provided by the cell's optics. These cells work well for absorption spectroscopy, but cannot be used to study gasses by Raman or fluorescence spectroscopy. Each pass through the White cell is distinct from all the other passes and there is no crossing point that could be the source of secondary emissions enhanced by multiple passes of light through said crossing point.
In order to use multipass cells for Raman, fluorescence, etc. studies of gasses a unipoint multipass cell was introduced [R. A. Hill, A. J. Mulac and C. E. Hackett, Retroreflecting Multipass Cell for Raman Scattering, Appl. Opt. 16 (1977) 2004-2008] that provided that all the passes cross in a single point. This crossing point of light is also the analysis point of the cell. A sample placed in this point interacts with all the passes through the cell greatly enhancing secondary emissions from this point. The unipoint multipass operation was achieved by two sets of retro-reflectors accompanied by two lenses. The midpoint between the lenses was also a focal point for the two lenses. Collimated light was retro-reflected back to the cell by the retro reflectors and refocused into the focal point by the lenses. By slightly offsetting one of the retro reflectors, the returning light is slightly offset with respect to the incoming light thus enabling multiple passes. After a number of passes, light falls out of the aperture of one of the lenses and exits the cell. The light intensity of every returning pass is reduced by reflection losses in the retro reflectors and on the lenses. Thus, after a number of passes, the intensity of the returning light is weakened sufficiently to offset the benefit of multiple passes.
Ducellier (U.S. Pat. No. 6,577,398B1) utilized the essential part of the Hill concept of a retro reflector and a lens to set up a resonant optical cavity. Only one set of a retro reflector and a lens arrangement of Hill was used and the multipass configuration was achieved by placing a beamsplitter into the focal point of the lens. The beamsplitter provided a partial return of the exiting light back into the cell therefore replacing the second set of a retro reflector and a lens from Hill.
Both Hill and Ducellier devices used a lens in combination with a retro reflector. All lenses suffer from spherical and chromatic aberrations. The spherical aberration of a lens is responsible for blurring of the image. All rays in a collimated beam entering a lens are not brought into a sharp point in the focus of the lens, but are spread over a small area around the focus. Lenses can be designed in a way that minimizes the size of the blur, but never to completely eliminate it. Repeated passes further degrade the imaging thereby limiting the number of passes that can be employed in the cell. Chromatic aberration of a lens is a consequence of the fact that the focal point of a lens is a function of the refractive index of the material that the lens is made of. Since the refractive index of materials changes with wavelength, the focal point of a lens is slightly different for different wavelengths of light. This means that for a beam containing multiple wavelengths of light, the cell can never be brought in alignment for all wavelengths simultaneously. Although techniques exist to minimize these aberrations, they bring in a high level of complexity and expense. The minimization of the aberrations can only be achieved over a relatively narrow range of wavelengths so a different cell has to be used for each range of wavelengths of light. In addition to the aberrations, reflection losses that light encounters in passing through the devices of Hill and Ducellier include four reflection losses on the lens and two reflection losses on the two mirrors of the retro reflector for each pass through the device. Again, techniques exist to minimize these loses, but they are effective only over a narrow range of wavelengths and add considerable complexity and expense.
A variation of the multipass cell configuration was introduced [J. C. Robinson, M. Fink and A. Mihill, New Vapor Phase Spontaneous Raman Spectrometer, Rev. Sci. Instrum. 63 (1992), 3280-3284] that utilizes two crossing points so that all the passes cross in one or the other point. Each of the points can become the source of Raman, fluorescence, etc. emissions. This cell design was an improvement on the unipoint multipass cell [Hill et al.] since it used only two spherical mirrors and thus had reduced reflectance losses. While the reflectance losses are reduced, they still limit the number of passes that can be effectively utilized by the cell. Also, having two crossing points instead of one reduces the gain achieved due to multiple passes.
Another version of the unipoint multiple pass concept has been proposed by Harrick [N. J. Harrick: Internal Reflection Spectroscopy, Harrick Scientific Corporation, Ossining N.Y., 1987.] for the ATR analysis of samples. This concept, however, was never reduced to practice because the shape of the ATR crystal required for the operation was too complex to manufacture and the optical design was not suitable for the reimaging of a typical spectrometer beam. However, it was recognized that if such a unipoint multipass cell could be developed, that it would be of great utility in ATR spectroscopy.
Thus there is a need for a system that can overcome the above and other disadvantages.